1. Field of the Invention
The present invention relates generally to a magnetoresistance effect element, a magnetic head and a magnetic recording and/or reproducing system. More specifically, the invention relates to a magnetoresistance effect element using a spin-valve film wherein a sense current flows in a direction perpendicular to the plane of the thin film, a magnetic head including the magnetoresistance effect element, and a magnetic recording and/or reproducing system including the magnetoresistance effect element.
2. Description of Related Art
There is known a phenomenon that an electric resistance varies in response to an external magnetic field in a certain kind of ferromagnetic material. This is called a “magnetoresistance effect”. This effect can be used for detecting an external magnetic field, and such a magnetic field detecting element is called a “magnetoresistance effect element (which will be hereinafter referred to as an “MR element”)”.
Such an MR element is industrially utilized for reading information, which has been stored in a magnetic recording medium, in a magnetic recording and/or reproducing system, such as a hard disk or a magnetic tape (see IEEE MAG-7, 150 (1971)), and such a magnetic head is called an “MR head”.
By the way, in recent years, in magnetic recording and/or reproducing systems utilizing such an MR element, particularly in hard disk drives, the magnetic recording density is being enhanced, and the size of one bit is decreasing, so that the amount of leakage flux from a bit is increasingly decreased. For that reason, it is necessary to prepare an MR element, which has a high sensitivity and a high S/N ratio and which can obtain a high rate of change in resistance even in a lower magnetic field, in order to read information which has been written in a magnetic medium, and this is an important basic technique for improving the recording density.
The “high sensitivity” means that the amount of change in resistance (Ω) per a unit magnetic field (Oe) is large. As an MR element has a larger amount of change in MR and a more excellent magnetically soft characteristic, the MR element has a higher sensitivity. In addition, in order to realize a high S/N ratio, it is important to reduce thermal noises. Therefore, it is not desired that the resistance itself of the element is too high, and when the element is used as a reading sensor for a hard disk, the resistance of the element is preferably in the range of from about 5 Ω to about 30 Ω in order to realize a good S/N ratio.
Under such a background, at present, a spin-valve film capable of obtaining a high rate of change in MR is generally used as an MR element for use in a hard disk MR head.
FIG. 19 is a conceptual drawing showing an example of a schematic cross-sectional structure of a spin-valve film. The spin-valve film 100 has a structure wherein a ferromagnetic layer F, a non-magnetic layer S, a ferromagnetic layer P and an antiferromagnetic layer A are stacked in that order. Of the two ferromagnetic layers F and P which are magnetically in a non-coupled state via the non-magnetic layer S, the magnetization of one ferromagnetic layer P is fixed by an exchange bias or the like using the antiferromagnetic material, and the magnetization of the other ferromagnetic layer F is set to be capable of being easily rotated by an external magnetic field (a signal magnetic field or the like). Then, only the magnetization of the ferromagnetic layer F can be rotated by the external magnetic field to change a relative angle between the magnetization directions of the two ferromagnetic layers P and F to obtain a large magnetoresistance effect (see Phys. Rev. B45, 806 (1992), J. Appl. Phys. 69, 4774 (1991)).
The ferromagnetic layer F is often called a “free layer”, a “magnetic field receiving layer”, or a “magnetization free layer”. The ferromagnetic layer P is often called a “pinned layer” or a “magnetization fixed layer”. The non-magnetic layer S is often called a “spacer layer”, a “non-magnetic intermediate layer” or an “intermediate layer”.
The spin-valve film can rotate the magnetization of the free layer, i.e., the ferromagnetic layer F. Therefore, the spin-valve film can be sensitized, so that it is suitable for an MR element for use in an MR head.
It is required to cause a “sense current” to flow through such a spin-valve element in order to detect the variation in resistance due to a magnetic field.
FIG. 20 is a conceptual drawing showing a generally used current supply system. That is, at present, there is generally used a system for providing electrodes EL, EL on both ends of a spin-valve element as shown in the figure to cause a sense current I to flow in parallel to the plane of the film to measure a resistance in a direction parallel to the plane of the film. This method is generally called a “current-in-plane (CIP)” system.
In the case of the CIP system, it is possible to obtain a value of about 10 to 20% as a rate of change in MR. In a shield-type MR head which is generally used at present, a spin-valve element has a substantially square shape, so that the resistance of an MR element is substantially equal to a value of plane electric resistance (sheet resistance) of an MR film. Therefore, a spin-valve film of a CIP system can obtain good S/N characteristics if the value of plane electric resistance is set to be 10 to 30 Ω. This can be relatively simply realized by decreasing the thickness of the whole spin-valve film. Because of these advantages, the spin-valve film of the CIP system is generally used as an MR element for an MR head at present.
However, it is expected that the rate of change in MR is required to exceed 30% in order to realize information reproduction at a high recording density exceeding 100 Gbit/inch2. On the other hand, it is difficult to obtain a value exceeding 20% as the rate of change in MR in conventional spin-valve films. For that reason, in order to further improve a recording density, it is a great technical theme to increase the rate of change in MR.
From such a point of view, in order to increase the rate of change in MR, there is proposed a spin-valve comprising a magnetic/non-magnetic layer stacked film wherein a pinned layer and a free layer are ferromagnetically coupled in a CIP-spin-valve (CIP-SV) film.
FIG. 21 is a schematic sectional view of a spin-valve film having such a stacked structure. That is, each of a pinned layer P and a free layer F has the stacked structure of a ferromagnetic layer and a non-magnetic layer. In the case of this structure, the scattering of electrons depending on spin in the magnetic layer/non-magnetic layer interface in the spin-valve film contributes to the MR effect. Therefore, if the number of the magnetic layer/non-magnetic layer interface between the pinned layer P and the free layer F is increased so that a larger number of conduction electrons pass through the magnetic layer/non-magnetic layer interface, it is possible to obtain a high rate of change in MR.
However, in the construction of FIG. 21, since the sense current I flows in parallel to the stacked structure although the number of interfaces increases, there is a strong probability that each of electrons will flow through any one of the layers, so that the number of electrons crossing the interface can not be so increased. Therefore, it is difficult to improve the high rate of change in resistance.
In addition, in the above described method, since the total thickness of the film increases by the non-magnetic layers which are stacked on the pinned layer P and free layer F, respectively, the value of resistance of the plane of the film, i.e., a so-called value of plane electric resistance (sheet resistance), greatly decreases, so that the value of change in resistance (=value of plane electric resistance×rate of change in MR) decreases. Since the output of the head is generally in proportion to the amount of change in resistance, there is also a problem in that the absolute value of the output decreases when it is actually used as a sensor.
For the above described reasons, also in the CIP-SV film having the multi-layer structure of pin and free layers shown in FIG. 21, it is substantially difficult to realize a high rate of change in MR exceeding 20% and a practical amount of chamber in resistance of 5 to 30 Ω.
On the other hand, as a method for obtaining a large MR exceeding 30%, there is proposed a magnetoresistance effect element (which will be hereinafter referred to as a CPP-artificial lattice) of a type (current perpendicular to plane (CPP)) that a sense current is caused to flow in a direction perpendicular to the plane of the film in an artificial lattice wherein magnetic and non-magnetic materials are stacked.
FIG. 22 is a conceptual drawing showing a cross-sectional structure of a CPP-artificial lattice type element. In a magnetoresistance effect element of this type, electrodes EL are provided on the top and bottom face of an artificial lattice SL comprising ferromagnetic/non-magnetic layers, and a sense current I flows in a direction perpendicular to the plane of the film. It is known that this construction can a good interface effect and a high rate of change in MR since there is a strong probability that the current I will cross the magnetic layer/non-magnetic layer interface.
However, in such a CPP artificial lattice type element, it is required to measure the electric resistance of an artificial lattice SL having the stacked structure of very thin metallic films in a direction perpendicular to the plane of the film. However, this value of resistance is generally very small. Therefore, in the CPP artificial lattice, it is an important technical theme to increase the value of resistance. Conventionally, in order to increase this value, it is necessary to decreases the junction area between the artificial lattice SL and the electrode SL as small as possible and to increase the number of stacked layers of the artificial lattice SL to increase the total thickness of the film. For example, when the element is patterned so as to have a size of 0.1 μm×0.1 μm, if a Co layer having a thickness of 2 nm and a Cu layer having a thickness of 2 nm are alternately stacked ten times, the total thickness of the film is 20 nm, and a value of resistance of about 1 Ω can be obtained.
For the above described reasons, in order that the CPP artificial lattice type film provides a sufficient head output to be used as a good reading sensor for a hard disk, it is necessary for the film to be the artificial lattice type, not the spin-valve type, from the standpoint of resistance.
However, when the MR element is used for an MR head, it is required to cause each of magnetic layers to be a single magnetic domain so as not to generate Barkhausen noises, while controlling the magnetization of the magnetic layer so that an external magnetic field can be efficiently measured. However, as described above, it is required to alternately stack many magnetic and non-magnetic layers in order to increase the value of resistance in the CPP-MR element, and it is technically very difficult to individually control the magnetization of such many magnetic layers.
In addition, when the MR element is used for an MR head, it is required to allow the magnetization against a small signal magnetic field to sensitively rotate to obtain a high rate of change in MR. For that purpose, it is required to improve the signal magnetic flux density at a sensing portion to obtain a large amount of rotation of magnetization even at the same magnetic flux density. Therefore, it is required to decrease the total Mst (magnetization×thickness) of layers wherein magnetization is rotated by an external magnetic field. However, in the CPP-MR element, it is required to alternately stack many magnetic and non-magnetic layers in order to increase the value of resistance. Therefore, Mst increases, so that it is difficult to improve the sensitivity to the signal magnetic flux.
For that reason, although it is expected that the CPP artificial lattice type film has a rate of change in MR exceeding 30%, it is difficult to sensitize the film in order to use the film as an MR sensor for a head, so that it is substantially impossible to use the film as the MR sensor.
On the other hand, it is considered that the spin-valve structure using FeMn/NiFe/Cu/NiFe, FeMn/CoFe/Cu/CoFe or the like adopts the CPP system.
FIG. 23 is a conceptual drawing showing a cross-sectional structure of a CPP-SV element. However, in such a CPP-SV construction, the thickness of a magnetic layer must be increased to about 20 nm in order to the value of resistance. Also in that case, it is predicted that the rate of change in resistance would be only about 30% at 4.2 K and about 15%, which is half thereof, at room temperatures.
That is, in the spin-valve film of the CPP system, the rate of change in MR is only about 15%, and the Mst of the free layer must be increased. Therefore, it is difficult to sensitize the film in order to use the film as an MR sensor for a head, so that it is substantially difficult to use the film.
As described above, although there are proposed various systems, such as the spin-valve film of the CIP system, the artificial lattice of the CPP system, and the spin-valve of the CPP system, it is difficult to realize a spin-valve film which can be used at a high packing density exceeding 100 Gbit/inch2, which has an appropriate value of resistance and a large amount of change in MR and which is magnetically sensitive, at present.